Moldboard plows and chisel plows have been provided with toggle link trip mechanisms that are biased with springs to over center positions to hold earthworking tools in groundworking positions. These toggle link trip mechanisms have a pair of links pivoted together with a pivot pin. Another pivot pin connects one of the links to a fixed structure mounted on the frame of the implement. The other link is pivoted to a standard carrying the earthworking tool. Springs connected to the links bias the links to an over center position. These trip mechanisms are used to allow the earthworking tool to swing rearwardly and upwardly when an immovable object is encountered by the tool. Adjustable stops have been used to adjust the amount of over center distance of the pivotal connection between the links. The prior art toggle links do not adjust the overall length of the linkage. They also do not have reliable adjustments whereby the amount of force necessary to trip the linkage can be varied, and accurately controlled.